During the operation of a utility grid providing electric energy to a number of consumers there may be grid fault or grid disturbance, including capacitor switching on-off events etc. which may lead to an overvoltage in the utility grid, i.e. to a voltage of the utility grid which is above a nominal voltage of the utility grid.
The overvoltage or switching transients may be in the range of 140% to 150% of the nominal voltage and may have a duration between 1 ms and 20 ms. The overvoltage of the utility grid in particular together with a high voltage induced by a switching action and/or a switching transient of transistor modules may result in extremely high voltages at some of the energy production facilities, such as a wind turbine. The transient overvoltage may destroy components of the energy producing facility, in particular the wind turbine, wherein the components may include filters, such as a pulse width modulation (PWM) filter, a converter and also auxiliary equipment, such as a UPS, etc.
The grid voltage may be determined based on measurements for example at the point of common coupling, where a plurality of wind turbines is connected to the utility grid. The overvoltage may for example occur several times per day, for example due to switching events on the transmission system connecting the point of common coupling to the utility grid or a number of consumers.
Conventionally, a number of wind turbines, such as a fraction of ⅔ may be disconnected from the utility grid due to the overvoltage in the utility grid. Thereby, these disconnected wind turbines can not anymore provide electric energy to the utility grid, thus reducing the efficiency of the wind turbine power plant. Switching off the wind turbines in a conventional system may also referred to as tripping the wind turbines.
EP 2 135 349 B1 describes a method for operating a wind energy system in case of overvoltage in the grid, wherein reactive power generated by the wind turbine is supplied to the utility grid, to lower the grid voltage.
However, it has been observed that conventional methods and arrangements for responding to an overvoltage in the utility grid are not satisfactory in all situations and in particular do not reduce the overvoltage in a reliable manner.